Current measuring shunt in lightning rods



Patented Apr. 4, 1950 CURRENT MEASURING SHUNT. IN: LIGHTNING RODS NilsiEl, Lindenblad; Rockyv Roint, ,N." Y., assignorr to RadiosCbrporationmf America a corporation.

of Delaware Appllcatlomllecemher 29, 19 44, Seriitliblinz-5702348 ductor discriminate between. inductive" voltage drop and: the? pure. resistance; drop. in said conduo-ton.

Anpobjecttherefore;--, of; the present: invention isrto provide at current 'measuring system which" is entirely: unresponsive; tow the inductive voltage dropin theawground conductor;

A further object oiythe present-invention isvto provide:- a mea-ns for investigating the: lightning discharge currents in# a. lightingr-rod located: on

theatoprof a high: building; said rod; being; so

constructed: as' to not: containafra-gilee insulators whichiifibrokenc would allow :fragrnentsrtot fall to the-street below to. the danger:offplersons' onthe street.

A- furtlierqobjecttoi the present'ainventi'on iS=l30 PYOVRlGFa-T system for me'asuring thelightning-adis; charge currentsin" a lightning: rod ground"? conductorvwherein' there HIBIIIO' insulating parts; the system beingentirelymetallic in" construction:

As further object of thepresent:- inventiorr is tmprovide a measuringequipment 1: of. they type mentioned above which is more-flexible; 1858: 635; pensive; safer, and-' more convenient than heretofdre k nown forms of consti'uctionr appear" from the following dtailed description are attained'by-providinga current shunt in the ground conductor connection, the shunt being -so constructed-that aninductivevoltage diop'across thesh'unt'will not appear :on*the*measuring-equip= ment connectedto the; shunt? This is" attained bye-providing a hollow shunt vin"whicli'at least" one .ofgthe conductors connectedito the: shuntpasses coaxlalli through the: centers of; the shunt.

Therefore-.thesame.magneticifield that'surrounds thewallsof the-hollow shunt surrounds thezcenter conductor; The wall 1 as-iwell :ascthezi-inside center conductorrare thus subjected to:,-thessamezeleetro:-

motive-domeeandrin the a same direction: This 1 causes; atibalancingronti action so; that; as; far as producing 'currentsrin the; inner *conductorris concerned; theefiectzis:equivalent ato no;:eleetromotive force beinggprodncedfrorrnthesontsideefieldg. Il'xe mire-resistive:dropzinsidestiiegshunttmayybesccm v 2; duetedithroug h 'aicoaxial :cable; to the desiredjype oi.-.measuring;equipmenta Thezpresent invention will :be-.=mor.e completely understood.ibwrefereneeato the following detailed description? which: is accompanied by: a. drawing in I which :the :lightning pickup rod I Dr consistss of a hollow tubular pipe directly grounded at; the point: of. mounting}: to; for example-an; antenna tower l 2: which is shown in fragmentary cross:- section. The pipe then continues insider the tower and, to thermeasuring equipment 13* as; an outer shell '22: of a coaxial transmissionlinei. Inisidethe lower portion l4 ofjthewprotruding-secetion-of the pickup-rod In, therezvista -meta'lplug I65 serving as one shuntterminal 1130 which; lead I8 is connected. Lead I flz'passesathrough :shellj22 formingthezinner conductor of the coaxial trans-'- mis'sionrline'. Transmission line- |8',,22' at the lower enditerminates in a resistorrlflhavingfa resistance, equal to; the surge impedance: of the coaxial line thus preventing thereflection'of energy' ba'ck upthe: line. Transmission line I8, 22; is further surrounded sbyz'a-secondouter 'shell 26 to reduce the resistance ofthe outer-shell 22 andto further sliieldthe measuringcircuit from-second;- ary undesired currents. The transmission. line, it'will be-noted, is entirely uncoupled from-any electro-magnetic"fieldsuset up from r azcurrentrby a lightning-discharge;

The: ohmic drop between the location of" the inside plug l6 and the*conneetion ottheipickup rodl0 toitheantenna tower -l2 shows: up: on :the inside ofii the; line. Ituis'. estimated thatf-ifthe lightningpickup rod' H1v consistswfia copperpipe of 1 inches in diameter with a sixteenth-.off'an inch wall thickness a: voltage,dropzof'i'six-tenths ofia, volt :would"be;produced across theshunt 'secstion I 4 if the:shunt section hasaan overall length of ten" inches and if the discharge currentiis of the:=order--of.5,000 amperes;v The voltage maybe increasedif necessary by usingthinnerwall1 sec? tions and/or: higher resistivity material to' increasetheresistance' of the shunt; Such an'irrcreasexin' resistance :mayalso be desirable tot-reiduce' possible error from skin' effect in the conductor;

The" measuring" equipment: [3, at: the: end; of the-transmission-line I8, 22; is preferablyentirely enclosed in metal box: 24: well 1 bonded r. to-outer= mosttshell '26 of the" transmission lines. The -in"- termediate shell isgroundedito theshielding'box through a non-reflecting resistor 28.

In' considering; the operation of "the invention, it should'be-znotednthat even though; due tothe steenzwavesfront ofitliercurrent im the lightning center conductor but they are effectively in seriesopposing relationship with respect to the measuring circuit so that they balance'out. The measuring equipment may include a film recording type of cathode ray oscilloscope to give a. permanent record of the lightning discharges. The

have not been shown since they are not included in the present invention. The oscilloscope, as is c'bnv'enti'onal, may include an amplifier with a linear characteristic to obtain the desired amplitude of deflection for the expected values of current flow through the ground conductor. In some cases it may be desirable to utilize a de- 'fiection amplifier so modified that low current discharges cause relatively larger deflections than high discharges, thus reducing the possibility of off-scale deflections for unexpectedly large discharges.

While I have illustrated a particular embodiment of the present invention, it should be clearly understood that it is not limited thereto since many modifications may be made in the several elements employed and in their arrangement without departing from the spirit and scope of the invention. The term ground as used hereinbefore should not be construed as being limited to an earthe'd connection but as including any point of fixed reference or zero potential.

What is claimed is:

1. A lightning rod having a ground conductor in the form of a hollow tube, an inner conductor coaxially arranged within said tube, a conduc tive plug connecting said inner conductor to said hollow tube at one position therewithin, a connection to another position within said hollow tube spaced a predetermined distance from said one position, and a measuring instrument connected from said inner conductor to said connection, the transmission line formed by said inner conductor and said connection being terminated by a resistor having a resistance equal to the characteristic impedance of said transmission line.

'2. A lightning rod having a ground conductor in the form of a hollow tube, an inner conductor coaxially arranged within said tube, a conductive plug connecting said inner conductor to said .hollow tube at one position therewithin, a connection to another position within said hollow tube spaced a predetermined distance from said one position, and 'a measuring instrument connected from said inner conductor to said connection, the transmission line formed by said inner conductor and said. connection being terminated' by a resistor having a resistance equal to thecharacteristic impedance of said transmission-line, and said measuring instrument being encased within a conductive shell connected to 'apoint of zero reference potential.

3. A lightning rod having a ground conductor in the form of a hollow tube, an inner conductor co'axially' arranged within said tube, a conductive plug connecting said inner conductor. to. said hollow tube at one position therewithin, a connection to another position within said hollow tube spaced a predetermined distance from said one position, and a measuring instrument connected from said inner conductor to said connection, the transmission line formed by said inner conductor and said connection being terminated by a resistor having a resistance equal to the characteristic impedance of said transmission line, and said measuring instrument being encased within-a conductive shell connected to a point of zero reference potential through a nonrefiecting resistor.

4. A- lightning rod having a ground conductor at least apart oi-which is in the form of a hollow tube, an inner conductor coaxially arranged within said tube, a conductive plug connecting particular details of the measuring equipment.

said inner conductor to said hollow tube at one position therewithin, a connection to another position within said hollow tube spaced a predeternii'rieddist'an'ce 'rmm' said one position, and a measuring instrument connected from said inner conductor to said connection, the transmission line formed by said inner conductor and said connection being terminated by a resistor having a resistance equal to the characteristic impedance of said transmission line, and said measuring instrument being encased within a conductive shell connected to a point of zero reference potential.

5. An"arrangement for measuring high current values in a circuit, including a hollowconductivetube, an inner conductor coaxially arranged within said tube, a conductive plugv connecting said inner conductor to said hollow tube at one point therewithin, a connection to another point within said hollow tube spaced a predetermined distance from said one point, and a measuring instrument connected from said inner conductor to said connection, the transmission line formed by said inner conductor and said connection being terminated by a resistor'having a resistance substantially equal to the characteristic impedance of said transmission line.

6. An arrangement for measuring high current values in a circuit including a conductor, a portionoiwhich is constituted by a hollow tube, an inner conductor within said tube, a conductive member connecting said inner conductor to said hollow tube at one point therewithin, a connection to another point within said hollow tube spaced a predetermined distance from said one point, and a measuring instrument connected from said inner conductor to said connection, the transmission line formed by said inner conductor and said connection being terminated by a resistor having a resistance equal to the characteristic impedance of said transmission line, and said measuring instrument being encased within a conductive shell connected to a point of zero reference potential.

7. An arrangement for measuring current flowing in a conductor in the form of a hollow tube, an inner conductor coaxially arranged within said tube, a conductive member connecting said inner conductor to said hollow tube at one point therewithin, a connection to another point within said hollow tube spaced a predetermined' distance from said one point, and a measuring instrument connected'fromsaid inner conductor to said connection, the transmission line formed by said inner conductor and said connection being terminated by a resistor having a resistance equal to the characteristic impedancelof said transmission line -and said measuring instrument being encased within a conductive shell connected to a point of zero reference potential through a non-reflecting resistor.

8. An arrangement for measuring current flowing in a conductor at least a part of which is in the form of a hollow tube, an inner conductor coaxially arranged within said tube, a conductive member connecting said inner conductor to said hollow tube at one point therewithin, a connection to another point within said hollow tube spaced a predetermined distance from said one point, and a measuring instrument connected from said inner conductor to said connection, the transmission line formed by said inner conductor and said connection being terminated by a resistor having a resistance equal to the characteristic impedance of said transmission line, and said measuring instru ment being encased within a conductive shell connected to a point of zero reference potential.

9. A structure adapted for measuring current impulses having steep wave fronts flowing through a conductor, including a hollow conductive tube connected in series with said conductor to constitute a resistance element through which the current flowing through said conductor passes, one end of said tube being at a point of fixed reference potential, a measuring instrument, and connections from each end of said resistance element to said measuring instrument, said connections comprising a section of coaxial transmission line having the sheathconductor thereof connected to the end of said tube at said fixed reference potential and having the center conductor thereof extending within said tube to the other end thereof, said section of coaxial transmission line being terminated by a resistor having a resistance value substantially equal to the characteristic impedance of said section, whereby the reactive effect of the resistance element to said steep wave front current impulses is substantially neutralized.

10. A structure adapted for measuring current impulses having steep wave fronts flowing through a conductor, including a hollow conductive tube connected in series with said conductor to constitute a resistance element through which the current flowing through said conductor passes, one end of said tube being at a point of fixed reference potential, a measuring instrument, and connections from each end of said resistance element to said measuring instrument, said connections comprising a section of coaxial transmission line having the sheathconductor thereof constituted by extension of said tube at the end thereof at said fixed reference potential and having a center conductor extending within said tube to the other end thereof, said section of coaxial transmission line being terminated by a resistor having a resistance value substantially equal to the characteristic impedance of said section, whereby the reactive effect of the resistance element to said steep wave front current impulses is substantially neutralized.

NILS E. LINDENBLAD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,232,179 King Feb. 18, 1941 2,273,547 Radinger Feb. 17, 1942 OTHER REFERENCES Pamphlet, Practical Analysis of Ultra High Frequency, R. C. A. Service Co., Inc., August 1943. 

